1. Field of the Invention
This invention relates to a conductive heater grid for use in defrosting plastic and glass panels, such as windows in vehicles.
2. Related Technology
Plastic materials, such as polycarbonate (PC) and polymethylmethyacrylate (PMMA), are currently being used in the manufacturing of numerous automotive parts and components, such as B-pillars, headlamps, and sunroofs. Automotive rear window (backlight) systems represent an application for these plastic materials due to their many identified advantages, particularly in the areas of styling/design, weight savings, and safety/security. More specifically, plastic materials offer the automotive manufacturer the ability to reduce the complexity of the rear window assembly through the integration of functional components into the molded plastic system, as well as the ability to distinguish their vehicles by increasing overall design and shape complexity. Being lighter in weight than conventional glass backlight systems, their incorporation into the vehicle may facilitate both a lower center of gravity for the vehicle (and therefore better vehicle handling & safety) and improved fuel economy. Further, enhanced safety is realized, particularly in a roll-over accident because of a greater probability of the occupant or passenger being retained in a vehicle.
In order to be used as a rear window or backlight on a vehicle, the plastic material must be compatible with the use of a defroster or defogging system, better known as a heater grid. For commercial acceptance, a plastic backlight must meet the performance criteria established for the defrosting or defogging of glass backlights. One difference between glass and plastics panels is related to the electrical conductivity exhibited by the heater grid. This difference in conductivity manifests itself in poor defrosting characteristics exhibited by the plastic window, as compared to the glass window. This difference in conductivity manifests itself in the inefficient heating of portions of the defroster, such as the busbar, that provides very little to no benefit to defrosting the overall window.
In addition to the previously mentioned drawbacks, the amount of electrical current traveling through each of the grid lines of the heater grid may vary. This variance causes grid lines with a less restrictive conductive path to heat up faster, leaving both defrosted and frosted portions of the plastic panels.
Therefore, there is a need for a system that will effectively defrost a plastic window with performance characteristics similar to that of a conventional glass window.